1. Field of the Invention
This invention relates to a process for incinerating solids on a water-cooled thrust combustion grate of the type installed, for example, in waste incinerators. This invention also relates to a specific grate plate and a grate having of a number of such grate plates for carrying out the process. The solids to be incinerated can be all kinds of different solids, for example lignite, shavings, chips of wood or rubber, residues of all kinds, industrial waste, sewage sludge, hospital waste or domestic refuse, and the like.
2. Description of Prior Art
In the case of conventional thrust combustion grates of the type installed in waste incinerators and which have layers of grates that rest on top of each other in the manner of a stairway, of which every second one can be moved in a thrust direction, primary air is blown from underneath and through the grate and into a combustion bed. In the case of cast grates, which are still the most widely used type of grate, where the individual layers of grates have of a row of cast grate bars positioned loosely next to each other or screwed to each other, the primary air reaches the upper surface of the grate through holes in the sides and/or the head portion of the cast grate bars. The primary air is blown through the grate by large ventilators in the zones underneath the grate which generate excess pressures equivalent to a column of water of the order of approx. 40 mm to 250 mm. Approximately 2% of each grate surface is reserved as a passage for the primary air, and the volume of air blown through can be up to 2,500 m.sup.3 of air per hour per square meter of grate surface. As the air flows through, it can reach peak speeds of over 30 m/s. This air that flows through the grate serves, on the one hand, as primary air for the fire, and, on the other hand, as cooling air for the cast grate. One of the disadvantages of this concept is that the penetration of the combustion bed by the air is very irregular. If, for example, a wire or any other small item lodges itself between two adjacent grate bars, the gap between them is widened at the cost of the gaps between the other grate bars. This means that the volume of air flowing through this gap will not be the same as the volume flowing through the gaps between the other grate bars. Another disadvantage is that, where the calorific value of the combustible material is high and the combustion bed is thin, as occurs repeatedly from spot to spot as the combustible material is transported along the flow of primary air breaks through the combustion bed at that point, creating a high darting flame which carries dust and ash with it far up into the boiler room without completely delivering all the oxygen to the fire. This causes a local excess of air, which has a negative impact on the flue gas.
A substantial improvement in the incineration process is achieved with water-cooled grates comprising hollow grate plates preferably made from sheet metal which advantageously extend over an entire width of the grate. The grate plates have primary air supply ducts, for example, primary air supply pipes that pass through the grate plate, possibly tapering towards the top, or the primary air supply ducts are formed by holes for blowing primary air through, so that the primary air can be blown through the grate from underneath and directed out onto its upper surface. Because the grate plates extend over the entire width, slag can no longer fall through the individual grate elements to end up underneath the grate, as can happen when the layers of grates are made up of a number of grate bars positioned loosely next to each other. This virtually eliminates the problem of falling slag. The great advantage of a water-cooled grate, however, lies in the fact that the air blown through it need only fulfil the function of supplying air for combustion, for example, need not fulfil any cooling function whatsoever. As a result, the volume of air needing to be supplied can be drastically reduced, leading to a much quieter and more efficient fire. The distribution of primary air across the individual primary air supply ducts remains largely even. One remaining disadvantage, however, is that, especially in the event of high calorific values and/or a combustion bed which is thin from spot to spot, the primary air flow exiting from a primary air duct opening located at such a spot can break through the combustion bed.
The overall requirements made of incineration processes are increasing constantly. Because the composition, and hence the calorific value, and also the volume of, for example, domestic waste fluctuates greatly from region to region and season to season, as do its physical characteristics such as specific weight, article size distribution, permeability to air, moisture, ash content, percentage of non-ferrous metals etc., it is not easy always to achieve good combustion of the combustible gases and slag while remaining within the values prescribed by regulations. One objective is to achieve an even distribution of temperature within the gas flow in the boiler room, for which purpose it is essential that the combustion process on the grate and in the furnace chamber above the grate is controlled and even. The finite number of primary air supply lines respective openings, the periodic blockage of individual openings, the irregular volume of loose material and the resultant differences in the heights of the layers of combustible material, plus variations in its calorific value, often lead, however, to uneven combustion.
An insufficient supply of primary air to air-cooled grates can cause the grate to overheat. The combustion zone is prolonged, leading to unsatisfactory combustion of the slag. The lack of air in the furnace chamber has a negative impact on the combustion of gas and on the flow patterns in the boiler room. This in turn leads to excessive soiling of the boiler walls. If individual primary air supply openings become blocked up, this leads to an increase in the speed of the air exiting from the other unblocked openings and, wherever the flow of primary air breaks through the combustion bed (blow-by), to the formation of streaks in the furnace chamber, increased formation of CO and NO.sub.x and an increase in dust emissions. If the nature of the combustible material causes total or partial blockages in the openings on one side of the grate, the combustion bed is rendered uneven, and the combustion process is only satisfactory on one side.